The present invention relates to a silicone dissolving agent and, more particularly, to a silicone dissolving agent that is thickened to remain in contact with a target silicone film.
Silicone rubbers are used extensively in electronic, construction, and automotive applications. Silicone rubbers have the attributes of solvent and high temperature resistance, and good adhesion properties to a variety of substrates. Silicone resins and rubbers upon curing are cross-linked polymers. Whereas silicone resins often find applications as electrical insulators and water repellant paints and finishes due to exceptional resistance to weather, sunlight, oxidation and high energy radiation, silicone rubbers, such as RTV silicones, most often find applications as seals and gaskets exposed to temperature extremes and limited classes of olefinic solvents.
Single component silicone rubber mixtures commonly used have good shelf lives and vulcanize at room temperature to yield elastomers. These mixtures generally include a polymeric, usually linear siloxane, a cross linker, a plasticizer such as methyl terminated polydimethyl siloxane and optional additives such as curing accelerators, pigments, processing aids and fillers.
Silicone rubbers and resins are labor intensive to remove and replace. Chemical silicone removers have achieved considerable popularity over abrasive methods such as sandpaper abrasive disks, since abrasion modifies substrate dimensions and finish. Additionally, abrasive grit residue often enters fluid circulatory systems and engine components where the silicone served as a sealant or gasket. Chemical silicone removers have generally been strongly acidic or caustic solutions that are not only able to digest cured silicone rubbers and resins, but also attack metallic substrates such as aluminum and steel. Extreme pH silicone removers have a deleterious effect of pitting metallic substrates and damaging wood substrates as well. Solvent swelling using organic solvents such as alkanols, toluene, methylene chloride and the like are capable of swelling a cured silicone rubber or resin yet still require mechanical abrading or scraping to remove the still cured silicone. Further, environmental concerns and the difficulty of maintaining volatile organic compounds in contact with silicone rubber have limited the utility of this method as well.
Silicone removers have become available based upon organosulfonic acid solutions. While organosulfonic acid solutions are effective in digesting cured silicone rubbers and resins, the high volatility and inability to spread thick layers of such a solution onto a silicone rubber or resin have limited the utility of these solutions in automotive and construction applications. Attempts to formulate a viscous silicone rubber or resin remover by mixing a sulfonic acid compound with a polymeric glycol diether and inorganic particulate as exemplified by Japanese published application 2000061390A have met with limited success owing to incomplete silicone matrix dissolution. Thus, there exists a need for a thickened silicone remover that can be applied to various silicone coated surfaces and remain in contact with the silicone until digested, the thickened silicone remover functioning without degrading the underlying substrate.
A silicone dissolving composition includes a sulfonic acid compound, a solvent miscible with the sulfonic acid compound, an organic or organometallic material thickener and sulfuric acid. The solvent is selected to swell a silicone matrix. A silicone dissolving composition is also disclosed including alone or in combination sulfonic acid and phosphinic acid, a solvent miscible with the organo-acid, and a thickener present in a concentration sufficient to maintain the composition in dripless contact with a silicone coated substrate until the silicone is dissolved. The solvent chosen is not only miscible with the organo-acid but also is able to swell the silicone matrix. A silicone film is removed by applying a composition according to the present invention to a substrate coated with a silicone film and allowing sufficient time for the silicone film to be dissolved by the composition.
The silicone rubber or resin dissolving agent of the present invention includes as an active ingredient from about 0.5 to 25 weight percent of an organo-acid compound, 25 to about 95 weight percent of a solvent miscible with the organo-acid compound and able to swell a cured silicone rubber or resin, a thickener present from about 2 to about 20 weight percent, and an amount of mineral acid present from about 0.025 to about 6% by weight total composition. Preferably, a silicone dissolving agent according to the present invention operates in less than two hours, and more preferably within 2-10 minutes, to digest a silicone to a consistency capable of being wiped from a substrate. Organo-acids according to the present invention include sulfonic acids, phosphinic acids, and phosphonic acids.
xe2x80x9cSiliconexe2x80x9d is defined herein to include polymeric silicone rubber or resin compositions which are cured or cross linked to form a polymeric matrix.
A sulfonic acid compound according to the present invention has the general formula R1SO3H where R1 is an aromatic group, or a C1-C24 alkyl or alkenyl or alkoxy group. Di- or tri- functional sulfonic acids are appreciated to be similarly operative herein. The aromatic group illustratively including phenyl, naphthyl, anthrocenyl, naphthylcenyl, penthacenyl, pyrenyl, phenanthronyl, heterocycles illustratively including pyrimidine, quinoline, isoquinoline, indole, imidazole, purine, furane, and thiophene. Preferably, the aromatic substituent is phenyl. Substituted aromatics operative in the present invention include replacement of an aromatic substituent proton with a group including C1-C20 aliphatics, alcohols, aldehydes, ketones, amines, imides and other heteroatom containing alkyl groups compatible with a hydrosulfonate. Preferably, a substituted aromatic sulfonic acid is a mono or dialkyl substituted phenyl such as dodecyl benzene sulfonic acid. A C1-C20 aliphatic substituent according to the present invention illustratively includes linear, branched, cyclic alkyls and alkenyls. It is appreciated that the choice of R substituent of a sulfonic acid according to the present invention is dictated by factors illustratively including solvent miscibility, silicone matrix interaction, storage stability, commercial availability, viscosity, and handling characteristics.
Optionally, a phosphinic or phosphonic acid having the formula R2PO2H2 or R2PO3H2, respectively, is utilized in the present invention in place of, or in combination with, sulfonic acid. R2 is a radical of coterminus scope with R1 as detailed with respect to sulfonic acid. Phenylphosphinic acid and phenylphosphonic acids are preferred phosphorus containing acids that are operative herein at levels as low as one weight percent.
A solvent miscible with a given organo-acid is chosen which is capable of swelling a cured or cross-linked silicone. Solvents according to the present invention include aliphatic and aromatic hydrocarbons that are liquid under normal storage and use conditions, illustratively including alkanes, aromatics, ketones, aldehydes, ethers, alcohols and esters. Preferably, a solvent according to the present invention has a limited odor and an evaporation rate less than about half that of n-butyl acetate at 20xc2x0 C. Owing to the gelled nature of the inventive compositions and rapid silicone dissolution, solvent volatility is of less concern than in prior art compositions. Solvents according to the present invention illustratively include petroleum distillate, hexanes, C1-C8 alcohols and toluene.
A thickener compatible with the silicone remover solvent is provided to promote adherence of a remover according to the present invention with a silicone coated substrate. Thickeners operative in the present invention illustratively include copolymers compatible in the remover solvent, dendrimers, emulsifiers, waxes, resins, inorganics and mixtures thereof.
In addition to the organo-acid utilized herein to dissolve a silicone, it is recognized that trace quantities of sulfuric acid or phosphoric acid typically present in commercial grades of sulfonic acid, and phosphinic and phosphonic acids, respectively, are active in the silicone dissolution process. Sulfuric acid is typically found in commercial grades of sulfonic acid in concentrations ranging from about 0.5 weight percent to about 2 weight percent. While this amount of sulfuric acid is often sufficient to facilitate silicone digestion within a matter of minutes, it is appreciated that sulfuric acid is optionally added beyond this level to modify silicone dissolving agent properties according to the present invention. Preferably, sulfuric acid is added to less than a concentration at which visible substrate pitting or other forms of degradation occurs in metallic substrates such as aluminum and stainless steel. Preferably, sulfuric acid is present at less than 3 total composition weight percent. Similarly, phosphoric acid which is present in phosphinic and phosphonic acids is maintained at levels of less than 3 total composition weight percent.
Optionally, dyes, fillers, wetting agents, defoamers, fragrances and other additives are included in the formulations of the present invention.